The present invention relates to an improved aqueous ink for ink-jet printing.
Generally, ink for use in ink-jet printing should satisfy several conditions in order to yield an excellent print, and there has been a keen demand for an aqueous ink which satisfies the following conditions:
First, in order that the ink be compatible with ink droplet formation and control of the direction of the ejected ink droplet streams, its viscosity, surface tension, specific electric conductivity, and density each fall within certain appropriate ranges. No precipitates are separated, due to chemical change or other causes, from the ink during an extended period of storage, or during the periods when the apparatus is not in use. No physical properties of the ink otherwise change during the above-mentioned periods. This is because, if the nozzles are plugged, for instance, with precipitates separated from the ink, it becomes impossible to eject the ink droplets from the nozzles, and even if the nozzles are not completely plugged, if solid components or viscous materials in the ink become affixed around the nozzles, or if the physical properties of the ink change, deviating from the predetermined physical properties achieved at the time of the preparation, the desired printing quality, the desired ink ejection stability, and the desired ink ejection response cannot be obtained.
Second, the ink provides adequately high contrast and clearness in the printed images.
Third, the images printed by the ink dries rapidly.
Fourth, the ink for ink-jet printing is such that the printed images are resistant to water, light and physical wear.
Fifth, the ink does not attack or corrode the container, tube, sealing materials, nozzles and valves and other parts of the ink-jet printing apparatus which come into contact with the ink.
Sixth, the ink is non-toxic to human bodies, free from unpleasant odor, and is non-inflammable.
As the conventional inks for ink-jet printing, C.I. Direct Black type dyes are widely used. However, when, for instance, C.I. Direct Black 19, which is one of the commercially available C.I. Direct Black-type dyes, is employed directly as the aqueous ink, it is not capable of meeting the above-described requirements.
More specifically, in the case of a conventional ink which is prepared by dissolving C.I. Direct Black 19 in an aqueous solvent, precipitates are separated, due to chemical change or other causes, from the ink during an extended period of storage, or during the periods when the ink-jet printing apparatus is not in use. As a result, the nozzles are plugged with the precipitates and it becomes impossible to eject the ink droplets from the nozzles.
Furthermore, since the solubility of C.I. Direct Black 19 in the solvent employed in the ink is low, the concentration of the dye cannot be increased. The result is that the ink cannot provide images with high density and high contrast.
In a conventional ink for ink-jet printing, in order to prevent the ink from drying within the nozzles when printing is not actually taking place, a comparatively large amount of a humectant is contained in the ink. This results in decreasing of the drying speed of the printed images.
In Japanese laid-open patent application No. 51-85804, there is proposed an ink for ink-jet printing, in which a chelating agent is contained in order to prevent accumulation of water-insoluble calcium salts in the nozzles, which causes the plugging of the nozzles. This chelating agent, however, brings about another problem that the chelating agent corrodes the metallic materials employed in the ink supply system of the ink-jet printing apparatus. Therefore, that ink is not suitable for practical use.
In Japanese laid-open patent application No. 54-120007, there is proposed an improved ink for ink-jet printing, in which a rust preventing agent is further contained in combination with the chelating agent disclosed in the above Japanese laid-open patent application No. 51-85804, in order to prevent the corrosion of the metallic materials employed in the ink supply system. However, the rust-preventing effect of the rust preventing agent is insufficient for practical use. In other words, the rust preventing agent is not capable of completely preventing the corrosion of the metallic materials. Further, the rust preventing agent precipitates and accumulates around the nozzles during the periods when the apparatus is not in use, and by the accumulated precipitates, the ink ejection direction is caused to considerably deviate from the desired direction.
Inks for ink-jet printing, containing a trisazo compound or a tetraazo compound, are also proposed. However, they are not suitable for practical use because of the poor solubility of those compounds in the solvent of the inks and because of incapability of yielding black images.
In addition to the above, so far, many proposals have been made for an ink for ink-jet printing, but from the viewpoint of practical use, a satisfactory ink which is capable of meeting the above-described requirements has not been found.